We have identified a rare subpopulation of human mammary epithelial cells (vHMEC) obtained from women with no known breast disease. These cells express a phenotype that allows them to enter a period of growth that is extremely vulnerable to mutagenesis and is typified by loss of specific cell cycle control and accumulation of a tremendous number of chromosomal abnormalities. Should these cells arise in vivo, they could be the origin of the initial lesions of human breast cancer. These observations identify novel opportunities, providing potential markers for assessing susceptibility to neoplastic transformation in individuals as well as potential targets for prevention and therapy. Multiple markers clearly identify the different cellular states and may allow the identification of these cells in vivo. Remarkably, the earliest lesions in breast cancer, hyperplasias, demonstrate abnormally controlled proliferation but relatively few chromosomal structural abnormalities, a phenotype similar to early-passage post-selection HMEC. The more progressed lesions in breast cancer, DCIS (ductal carcinoma in situ), additionally demonstrate the types of chromosomal aberrations observed in late-passage HMEC. We hypothesize that the above-described properties of HMEC in vitro are critically relevant to their transformation processes in vivo. We hypothesize that this in vitro model system will not only yield insights to the earliest steps in carcinogenesis in breast cancer, but will also allow us to identify and test new agents for prevention based on the molecular features of the HMECs as they transform. We hypothesize that the frequency of these cells in high risk women will be a possible marker for targeted prevention studies and postulate that agents that selectively kill post-selection HMEC should decrease the incidence of breast cancer. We will use this newly acquired data to take our SPORE project in a new direction and devise a model system for studying risk assessment and chemoprevention of breast cancer in vivo. The general goals of this new effort are to use this system to identify molecular characteristics of post-selection HMEC as endpoints to monitor the potential for malignant transformation activity and then use these to evaluate candidate chemopreventive agents. We will achieve this by (1) determining the frequency, distribution and defined molecular characteristics of vHMEC in vivo in women at low risk for developing breast cancer, (2) determining if the frequency, distribution and defined molecular characteristics of vHMECs in tissues from individuals at high risk for breast cancer are different as compared to tissue from low risk individuals, and (3) identifying agents which will selectively kill vHMECs in vitro and providing information to test their utility as preventive agents in vivo.